Rectifiers of different designs may be used for supplying direct current systems from three-phase current systems. Bridge rectifiers in a six-pulse version are frequently used in motor vehicle electrical systems in accordance with the three-phase current systems typically installed therein. However, the present invention is similarly suitable for bridge rectifiers for other numbers of phases, for example, for five-phase generators.
One critical fault in the case of bridge rectifiers is load shedding (load dump). This occurs if, in the case of a highly excited generator and a correspondingly high emitted current, the load on the generator or the bridge rectifier connected thereto (for example, due to shutdown of consumers) is reduced suddenly and the load cannot be absorbed by capacitively acting elements in the DC voltage network (for example, the battery in the motor vehicle electrical system). In this case, energy could still be supplied into the motor vehicle electrical system by the generator or the bridge rectifier connected thereto, in the extreme case up to a period of approximately 300 ms to 500 ms. This energy has to be able to be absorbed in the bridge rectifier to protect electrical components in the motor vehicle electrical system from overvoltage damage. This is generally carried out in passive bridge rectifiers by the rectifier diodes installed therein, in which the excess energy may be converted into heat.
As explained in German Patent Application No. DE 10 2009 046 955 A1, for example, the use of active bridge rectifiers is desirable in motor vehicles, however, inter alia, because they have lower power losses in comparison to passive or uncontrolled bridge rectifiers. Presently available activatable or active switching elements for such active bridge rectifiers, for example, MOS field effect transistors, are not able to dissipate overvoltages like diodes, however. Therefore, additional protection strategies are required in active bridge rectifiers.
In the event of load shedding, for example, the generator phases may be briefly short-circuited by switching all switching elements of the upper or lower rectifier branch to be conductive, as disclosed, for example, in German Patent Application No. DE 198 35 316 A1 and discussed in German Patent Application No. DE 10 2009 046 955 A1. This takes place in particular on the basis of an analysis of the output voltage applied to the DC voltage terminals of the active bridge rectifier. If it exceeds a predefined upper threshold value, a corresponding short-circuit is initiated and the output voltage drops. If the output voltage thus falls below a predefined lower threshold value, the short-circuit is canceled again. The output voltage rises again. It is therefore typical hysteresis behavior. The output voltage therefore generally swings between the upper and the lower threshold values in the event of load shedding, until the voltage regulation has adapted to the new situation and has accordingly reduced the exciter field of the generator.
Problems may arise here in so-called decentralized active bridge rectifiers, in which the individual half-bridges each have independent control circuits, which each acquire the output voltage individually. Since certain tolerances are unavoidable in this case, different switching behavior may take place in the individual half-bridges, as explained below. Hence, individual switching elements in the active bridge rectifier may be significantly overloaded, which may result in thermal destruction of the corresponding switching elements and a failure.
The demand therefore exists for improved protection strategies for active bridge rectifiers in the event of load shedding.